1. Field of the Invention
The present invention relates generally to velocity measuring apparatus and more specifically to a low cost portable Doppler-type meter for use in rapidly and accurately ascertaining the relative velocity of an object.
2. Description of the Prior Art
Radar, an acronym for RAdio Detecting And Ranging, has been employed for many years as a navigational aid. Traditional radar systems make use of the known propagation rate of microwave energy to ascertain the distance from the radar to an object, the distance being proportional to the time required for microwave energy to propagate from a source to the object and for resultant reflected energy to return.
Some specialized systems alternatively or additionally, measure the relative velocity of an object. The relative velocity is the approaching or receding speed of the object. Such measurements rely on the frequency shift called Doppler shift which microwave energy undergoes when it is reflected from an object having a relative velocity with respect to the source. Such a shift is quite pronounced. For an object illuminated by 10.5 gigahertz X-band energy, the shift is of the order of 31 hertz for each mile per hour of relative velocity.
Typical portable or semi-portable prior art systems of this latter class are tbe Doppler-type speed meters commonly employed by law enforcement personnel to monitor the relative velocity of motor vehicles.
Early speed meters are of the continuous wave, or CW type. They employ a continuously operating microwave energy source to generate energy. A major portion of the energy is radiated so as to form a beam that may be directed along a highway. Another portion of the energy is used to bias a microwave detector.
When a motor vehicle enters the beam, a fraction of the energy illuminating the vehicle is reflected back to the source where it is mixed in the detector with the biasing energy. As a result of the mixing process, a new difference signal is generated which has a frequency equal to the difference between the frequencies of the transmitted energy and that which is received. In other words, a signal is generated having a frequency which is proportional to the velocity of the motor vehicle. This difference signal is amplified and its frequency ascertained in order to develop a visual display indicative of the relative velocity of the motor vehicle.
Throughout the years, these prior art speed meters have evolved. Although such evolution is obviously due in part to advances in technology, no doubt the primary reason for such evolution is due to the need to counteract, counter-measures taken by motorists. As motorists became more wary and employed counter-measures such as receivers tuned to radar frequencies, speed meters changed. Early meters, which because of their bulk were designed to operate from the open trunk of a vehicle, were made smaller and semi-portable and the frequency of their radiated energy sources and its polarization were changed to defeat the receivers.
To further defeat radar receivers, recent prior art speed meters have incorporated a hold mode. In the hold mode, the microwave energy source may be temporarily disabled until a motor vehicle approaches. At this time, the operator may return the meter to the regular CW mode to ascertain the speed of the vehicle.
Although effective in measuring the relative velocity of motor vehicles, these prior art meters are ill-suited for other uses such as measuring the velocity with which a tennis player serves a tennis ball or a pitcher throws a baseball. Obviously, the high cost of these prior art meters limits such use; however, they also suffer from more fundamental weaknesses. Noise in the form of energy reflected by extraneous moving objects, if significant at all, merely acts to shorten the range at which prior art meters are effective. In most cases, the range may be increased by increasing the directivity of the speed meter antenna to both increase the level of the energy striking the motor vehicle and to reduce that striking other moving objects.
Not only do baseballs and the like have a significantly smallar radar cross section and thus, reflect less microwave energy, but also the energy they reflect is much more difficult to differentiate from noise. Obviously, increases in antenna directivity are ineffective to separate energy reflected by a pitcher during wind-up or follow-through from that reflected by the ball.
Timing of the measurement is also of great significance. If the measurement starts before the ball is pitched or completed after the ball is caught, an interval which may be less than one half of one second, the measurement will be inaccurate. Further, for most accurate results, the measurement should be completed within a fraction of this interval. This is desirable both because the speed of the ball is decreasing and because, in most cases, the angle of incidence of the microwave energy with respect to the velocity vector of the ball is increasing.